The present invention relates to a device for operating and controlling the needle in a sewing machine of the type having a needle supporting clamp which is mounted and guided for sliding movement on a stationary needle bar. The needle bar is mounted on the frame of the machine and pivotably connected to one end of a control lever that is pivotably supported intermediate its ends on a crank which forms the end of an oscillatably driven shaft of the machine. The opposite end of the control lever is pivotably connected to one end of a moveable lever element whose opposite end is also pivotably supported by the frame of the machine.
Devices of this type are well known in the art and are utilized because their particular design ensures low loading on the support bearing and on the pivot elements. This is attributed to the fact that the needle-supporting clamp has its fulcrum on that end of the control lever which is caused to move along a path which is substantially rectilinear and the needle bar is carried by the frame of the machine in a manner to effect a secondary function of serving as a means for movement of the needle about the fulcrum which interconnects said control lever with the needle-supporting clamp.
However, a number of disadvantages have been experienced with these known devices the disadvantages deriving from the fact that the needle bar is mounted on the frame in an operative position which is statically determined (i.e. the machine is not operating) and from the fact that the path through which the control lever passes is not a true rectilinear path. The path is, rather, similar to a double very elongated "S", the width of which is about 1 hundredth of a millimeter. These disadvantages or drawbacks are accentuated by the additional fact that each device has its own unique dimensions which differ from one another due to the tolerances which differentiate the individual components from each other and as a result, with each operating device mounted on a machine frame, it is necessary to separately determine the most desirable operating position of the needle bar relative to the path through which the needle-supporting clamp travels so as to reduce friction between the surfaces in sliding contact and consequently to reduce wear thereof.
The positioning of the needle bar with respect to the operating device is determined empirically by displacing and inclining said needle bar in its assembled seat until a position is found at which sliding of the needle-supporting clamp can be accomplished with a minimum of resistance.
Consequently, this manner of positioning depends very much on the skill of the person employed for the assembly of the machine, as a result of which even the wear of the two elements coupled together is influenced by a human factor.
It was further found that the needle-supporting clamp bears the major part of the negative effects of rubbing against the needle bar and that the most clear evidence of wear is found in particular at the extreme and opposing faces of the hole in said supporting clamp within which said needle bar is disposed.
This condition is due to the fact that a poorly positioned needle bar reacts to the stresses caused by the needle-supporting clamp particularly at points corresponding to the extreme points of its path since the double "S" has its greatest deviations at these extreme points.
A further negative factor encountered in known operating devices, and which favored wear of the needle clamp, is caused by the centrifugal force generated by the crank that is fixed to the oscillating shaft of the machine and pivotably connected at a central location on the control lever.
This crank, which as is known, is disposed in spaced relation to the needle bar and commences to generage a centrifugal force as soon as it starts to oscillate, and a large amount of force is transmitted to the movable lever element of the pivoting arrangement thus causing the needlesupporting clamp to place a greater load on the needle bar. Wear of the needle carrying clamp creates an increase in the amount of clearance or play between the hole provided in it for sliding movement on the needle bar and consequently leads to a gradual reduction of the loading forces thereon. However, this increase in clearance reduces the effectiveness of the specific function of the needle bar which is that of maintaining the operative phasing between the needle and the lower stitching instrumentalities of the sewing machine.
An object of the present invention is to eliminate the above described negative factors, the influences of which affect the efficiency of the complete operating device and of the needle-carrying clamp in particular.